Bottom currents and bottom sediment distribution in Massachusetts Bay

Abstract

Massachusetts Bay is a coastal Bay 100 km long and 40 km wide located in the western Gulf of Maine. The Bay is open only to the Gulf to the east; the opening is partially blocked by a shallow bank. The bottom sediment distribution in the bay is complex; fine grained material is found in the deep basin, sand and gravel on the shallow bank, and mixtures of sand, gravel and finer sediments nearshore. Richardson current meters were moored l m from the bottom over a one-year period at several locations in the Bay to define the bottom currents and to determine the frequency of bottom sediment movement due to currents. The measurements suggest that the bottom sediments can be expected to move occasionally in certain areas. The maximum bottom current speeds were determined principally by the strong tidal currents in the basin. In winter, the near bottom currents were dominated by wind stress associated with storms. Bottom currents in the shallow areas were generally in the direction of the wind while currents in the deep portion of the basin were often opposite to the direction of the wind. Sea-surface setup in the direction of the wind was observed, as well as variations in sea level attributed to changes in the adjacent Gulf of Maine. Adjustment of the bottom current to windstress required approximately 12 hours. Total concentrations of suspended particulate matter, in general, were found to be higher in the near-shore and near-bottom waters and lower in the offshore and near-surface waters. Near-bottom concentrations tended to increase as the grain size of the bottom sediments decreases. The greatest concentrations of suspended matter were observed in waters adjacent to Boston Harbor.